1. Field of Invention
This invention relates to a hand-held pneumatic power tool to be used for delicate hand engraving and stone setting in the hand engraving and jewelry field.
2. Description of Prior Art
Traditionally the tool used (and for a large part still used) for hand engraving and stone setting is a palm push graver tool. This traditional hand engraving tool consisted of the working point or graver set into a wood handle that fit comfortably into the palm of the hand. I have been a full time hand engraver for twenty years and find this traditional palm push graver is the most comfortable. A problem that arises with this traditional tool is that as the graver point is pushed through a cut, even in very fine engraving cuts, there is a small loss of control due to the force that is exerted. The outcome of this loss of control can be the graver point exiting out of the cut and the exerted force on the tool will cause slippage across the work. For heavier engraving cuts, engravers and stone setters have used a small hand-held hammer to strike against the graver to drive it through a cut. This has helped with some of the problem described above, as it is unnecessary to exert a great deal of force when the hammer does the work. A disadvantage with this hand-held hammer method is it leaves the engraving cuts jagged with small flats caused by each hammer impact.
In recent times impact power tools have been developed to attempt to aid the jeweler and engraver. For example, U.S. Pat. No. 4,694,912 (1987) and U.S. Pat. No. 5,203,417 (1993) both to Glaser, use compressed air through a rotary valve to generate pulses of air. This is used to move a piston in the hand-held device forward, depressing a spring and at the same time impacting the graver holder. This design has an adverse effect of loss of power or no power at all when the piston floats, caused when the pulses of air do not give the return spring time to return the piston. This is caused by the frequency of the air pulses being too close together and/or by too much air pressure in each pulse. Moreover, previous hand-held engraving impact tools such as U.S. Pat. No. 3,393,755 to Glaser and Rohner (1968) have required a separate, specialized source of vacuum air pulses to the hand-held device. In the case of U.S. Pat. No. 4,694,912 to Glaser (1987) described above, a specialized rotary valve is required as a means of power to rotate the valve to provide a source of air pulses to the hand-held impact device.
A more recent patent U.S. Pat. No. 5,515,930 to Glaser (1996) discloses a hand-held pneumatic apparatus. This hand-held impact tool also uses a spring, similar to the impact tools described above, for the return stroke of the piston. This spring has an unfavorable effect to the range of impacts that can be achieved with the device, as enough air pressure must be used to compress the spring sufficiently to enable the piston to come into contact with the graver holder or anvil. A spring that is very light in strength can be used and finer impacts will be achieved, but this results in insufficient high impact power when the user desires greater impact energy. On the other hand, if a stronger spring is used, air pressure will need to be increased to supply enough force to depress the heavier spring and this additional pressure will cause the piston to travel with more velocity and consequently the user discovers that the tool cannot achieve fine low power impacts, but only high power impacts. It should be noted that this described strength of spring and range of impacts problem also exists with impact tool disclosed in U.S. Pat. No. 4,694,912 to Glaser (1987). Returning to U.S. Pat. No. 5,515,930 to Glaser (1996), this device also requires a special pressuresensing element within the foot valve to start the piston oscillating by giving a quick surge of higher air pressure. This is needed as the tool's housing and tip will vibrate excessively if the tool housing or tool tip is not held against a fixed work surface while the device is oscillating and therefore requires the tool to start after the tool tip is placed against the work with the surge of higher air pressure. This vibration on an operator's hand can quickly fatigue the hand and can also make it difficult and impractical to place the tool tip where desired to begin an engraving cut. In addition, the vibration can dull the tool tip or ruin the work if the user attempts to place the tool tip to the work while the device is oscillating. This tool therefore requires its tip to be set into the work before starting the oscillation and therefore requires the special sensing element to help it start with a surge of air pressure. The air pressure surge to start the oscillation can also cause a surge of harder impacts than desired, making fine engraving work impractical.